1. Field of the Invention
The present invention relates to a measurement apparatus which measures the distance between a reference surface and a test surface.
2. Description of the Related Art
A wavelength scanning interferometer is known as a measurement apparatus which measures the distance between a reference surface and a test object. The wavelength scanning interferometer obtains the distance between a reference surface and a test object based on a temporal change of the intensity or phase of interference light caused by temporally scanning the wavelength of light emitted by a light source. The measurement accuracy of the wavelength scanning interferometer increases for a larger wavelength scanning range. The maximum measurement range depends on the coherence length of light emitted by a light source. To measure a long distance, a single mode laser is preferably used to generate light with a long coherence length.
Literature 1 (KUO Chih-Che et. al., “Signal Processing for Wavelength Scanning Interferometer”, Journal of the Japan Society for Precision Engineering, The Japan Society for Precision Engineering, Vol. 69, No. 6, p. 831, 2003) reports a wavelength scanning interferometer which obtains a distance by scanning the wavelength at a width of about 100 nm, performing FFT processing for the intensity of interference light at each wavelength, and detecting the peak of a modulated frequency. In literature 1, a measurement range of 1.56 mm can be measured at a resolution of 0.06 μm by improving the peak detection accuracy after FFT to 1/100 of the FFT pitch by interpolation or the like.
An FM heterodyne method, which is one of measurement methods for the wavelength scanning interferometer, measures the intensity of an interference signal at a fixed wavelength, and calculates a distance from a change of the intensity of the interference signal caused by wavelength scanning. For example, literature 2 (Japanese Patent No. 2725434) discloses a technique of guaranteeing the scanning amount of the wavelength using a reference interferometer (that is, using the length of the reference interferometer as a reference), and guaranteeing a fixed wavelength using a wavelength reference such as an etalon or gas cell in the FM heterodyne method.
However, when the wavelength scanning interferometer tries to obtain a desired measurement accuracy, measurement range, and measurement speed, it needs to use a light source which simultaneously satisfies a desired wavelength scanning range, coherence length, and wavelength scanning speed. This greatly restricts selection of a light source in the arrangement of the wavelength scanning interferometer.
The wavelength scanning interferometer reported in literature 1 obtains high measurement accuracy because of a wide wavelength scanning range of 100 nm. However, the measurement range is as narrow as about 1.6 mm owing to a short coherence length of the light source.
Some external cavity semiconductor lasers are known to oscillate in the single mode and have a long coherence length. When a wide wavelength scanning range is obtained, such a semiconductor laser cannot often achieve high-speed wavelength scanning and is expensive. Even an inexpensive distributed-feedback laser (DFB laser) can be used to obtain a wide wavelength scanning range by wavelength scanning based on temperature modulation. However, following takes time, so wavelength scanning cannot be done quickly. Wavelength scanning can be performed quickly by changing a current to be supplied to a vertical cavity surface emitting laser (VCSEL) or DFB laser. In this case, no wide wavelength scanning range can be obtained.
The technique disclosed in literature 2 needs to widen the wavelength scanning range to improve the accuracy, and the cost of the light source rises. This technique requires a reference interferometer, complicating the arrangement of the interferometer. In addition, the measurement accuracy decreases upon variations of the length of the reference interferometer serving as a reference.